Diesel engines, which inject into a cylinder high-pressure fuel gas, such as CNG (compressed natural gas) whose pressure is increased by gasifying LNG (liquefied natural gas), are known as, for example, gas fired low-speed diesel engines (SSD-GI; Slow-Speed Diesel-Gas Injection), and are regarded as being promising as main engines for propelling next-generation LNG ships and LNG/LPG fuel ships. The high-pressure fuel gas has a normal temperature (30 to 50° C.), and the pressure thereof is required to be adjusted to a range of about 150 to 250 bars (300 bars at the maximum) in accordance with the load of the diesel engines.
As the methods of producing the high-pressure fuel gas, there are generally two types. One is a liquid compression type (for example, refer to PTL 1) in which the temperature of LNG is raised to the normal temperature after the pressure of the LNG is increased by a liquid pump, and the other is a gas compression type in which the pressure of the gas after LNG has evaporated is made high by a compressor, and the high-pressure gas is cooled to the normal temperature with clean water or the like and is guided to a diesel engine.